Carbamazepine has been shown to increase plasma phenytoin levels in epileptic patients receiving both drugs. The rise in phenytoin plasma levels can lead to nystagmus, sedation, and other central nervous system side effects. The use of an in vitro system of hepatic microsomes can be used in elucidating the biochemical mechanisms of this drug interaction. Microsomes will be obtained from rats, mice, and rabbits. The Michaelis-Menton (Km) and maximal velocity (Vmax) will be determined for each species by following the rate of formation of HPPH, the major metabolite of phenytoin. This enzymatic process will be followed at 37 degrees in a NADPH-NADP system. The species which appears to have a similar Km, Vmax and metabolic profile to humans will be used for this study. The type of inhibition (competitive, noncompetitive) and the inhibition constant (KI) for carbamazepine will then be determined.